The present invention relates to polyolefin stretch films. In particular, the present invention relates to flexible films which may be used as a substitute for poly(vinyl chloride) (PVC) flexible films.
Poly(vinyl chloride) is a thermoplastic polymer which has been used extensively for many years to form articles, containers and films for industrial, commercial and consumer applications. Plasticized poly(vinyl chloride) has been used to make flexible films (hereinafter "PVC film") which have enjoyed great commercial success in both nonfood and food contact applications. In particular, PVC film is in widespread use as a food overwrap, e.g. for fresh fruits and vegetables, cheese, sandwiches, food trays and especially for fresh red meats, processed meats and poultry. PVC film is also used to package frozen foods, baked goods, fresh fish, as institutional packaging for overwrapping both food and such nonfood items as glassware and eating utensils, and as pharmaceutical packaging, pallet wrapping, and even as a laminate film for book covers. PVC film is well known for its stretchability, cling, clarity, transparency, gloss, toughness, machinability, heat sealability, resiliency and low cost. PVC film is used for packaging foods on trays in which the film overwraps the food and is generally tacked or sealed to itself at the bottom of the tray by heat. PVC film used to package items such as fresh red meat also has the high oxygen permeability needed to develop the bright red color that fresh red meat forms when contacted with oxygen. In addition, these overwrapped trays use PVC film having excellent elastic recovery from deformations such as those caused by depressing a thumb or finger against an overwrapped cut of red meat, poultry or ground meat. The properties of PVC film are typically easily modified for particular applications by adjusting the film thickness, or the type or amount of plasticizers and additives such as antifog, antiblock or slip agents.
With all of the above advantages, the search for a polyolefin based, thermoplastic, flexible film which may act as a substitute or replacement for PVC film has been difficult. No monolayer film having an equivalent or superior combination of properties has been found. Several different polyolefin stretch films have been commercialized to varying degrees of success. Multilayer polyolefin films have come the closest to achieving a combination of physical properties which approximate those of PVC films. Such polyolefin multilayer films generally sell at a premium above the price of PVC film used for similar applications. A market has developed for chlorine-free films having a combination of properties similar to PVC film and which may act as a substitute for PVC films in various applications.
There have been prior efforts to identify a polyolefin thermoplastic film having an improved combination of elongation, elastic memory, heat sealability and puncture resistance. However, most thermoplastic polyolefin film packaging materials suitable for food contact have relatively poor combinations of properties particularly with respect to elasticity or elastic memory.
Another important film property for food tray overwrap applications is low permanent deformation, which is a measure of the film's increase in length after stretching and relaxation. More particularly, the permanent deformation test used herein is derived from ASTM D-621 and measures the percent increase in length of a film sample stretched 50% and then allowed to recover for 30 seconds. Lower percentage values reflect the greater ability of a film to recover after being stretched. In tray overwrap applications, good film recovery preserves package appearance and integrity following handling abuse. Perfect elastomers would have 0% permanent deformation. Plasticized PVC has 3-5% deformation, while polyolefins such as linear low density polyethylene (LLDPE) generally have significantly higher permanent deformation, on the order of 6-15%. Thus, when a food wrapped in a poor deformation recovering material is poked by a consumer to test for freshness, the film does not immediately return to its former state and a deformed package having a depression caused by the finger poke results, which is aesthetically unpleasing.
Commercialized multilayer polyolefin stretch films are available under the trademarks Yuka Wrap from Mitsubishi and Aliprot from Crocco.
Yuka Wrap is described in four page brochure from Mitsubishi Petrochemical Co. Ltd. as an "environmentally sound/safe/non-PVC film" that has no plasticizer, yet provides ample extendibility and good restoration, a wide sealing range, well-balanced cling and slipperiness for use in high speed wrapping machines, and good optical properties.
Aliprot is described in a 13 page brochure from Crocco as "the recyclable film" which is polyolefin-based containing no chlorine and no plasticizers and which is suitable for food cling wrap applications. Aliprot film purportedly is suitable for use with automatic wrapping machines and has high puncture resistance and good printability.
Disadvantageously, both Yuka Wrap and Aliprot cling wrap films have poor elastic recovery from finger pokes when compared to PVC film.
It would be desirable to provide a multilayer, flexible, stretchable blown film having a combination of physical properties making it suitable as a substitute or replacement film for PVC film in food contact applications.
It would be advantageous for such film to be transparent, clear, have low haze and high gloss in combination with having a high degree of deformation recovery, rapid elastic recovery, good oxygen permeability which is sufficient to facilitate red bloom in fresh meat, heat sealability and good machinability.
One object of this invention is to provide an improved multilayer stretch film suitable for use as a trayed food overwrap material.
A further object is to provide such a stretch film in the form of three or more layers which has excellent puncture resistance and elastic recovery.
Another object is to provide a polyolefin-containing multiple layer stretch film having low permanent deformation.
Another object of this invention is to provide such a film which is capable of forming a sealed trayed food package as a stretch overwrap material.
Still another object is to provide such a film having high abuse resistance.
Yet another object of the invention is to provide a film which may be used on automatic packaging machinery.
A further object of the invention is to provide a film in roll form which may easily be cut for use in packaging, particularly on automated equipment.
An additional object is to provide such a film which can be heat sealed as a stretch wrap material around a food-containing tray at low temperature and energy.
A further additional object of this invention is to provide an improved heat sealed food-containing tray package overwrapped by a film having good deformation recovery.
Another additional object of the invention is to provide a chlorine free, polyolefin stretch film that is substantially free of heat shrinkage at 90.degree. C., but having a combination of excellent extensibility, finger poke recovery, optical properties, and heat sealability.
Yet another object of this invention is to provide a film which achieves one or more of the above objects and which may be made by a blown film or slotcast process.
These and other objects and advantages may be found in various embodiments of the present invention. It is not necessary that each and every object or advantage be found in all embodiments of the present invention. It is sufficient that the present invention may be advantageously employed.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.